1. Field of the Invention
The present invention relates to Magnetic Resonance (MR) image displays, and more particularly, to an auto-windowing system for MR images capable of adapting to optimal display parameters, personal preferences and different viewing conditions while online.
2. Prior Art
Adaptive and automatic adjustment of the display window parameters for Magnetic Resonance (MR) images under different viewing conditions is a very difficult and challenging problem in medical image perception. There are several factors that make this problem difficult, namely, the function describing a human expert's adjustment of display window parameters for a wide variety of MR images is extremely complicated, the adjustment result is subjective and substantially depends on personal preference, the adjustment function varies with the viewing conditions, etc. The viewing conditions are generally the condition of the monitor and the exterior viewing environment, such as the illumination condition. It is almost impossible to account for all these issues in a single universal algorithm.
The display windowing process is primarily used in the mapping of 12-bit MR image intensity values into 8-bit gray levels for displaying MR images on a common 8-bit computer monitor. The display window consists of a width parameter and a center parameter. The windowing process maps the image intensity values linearly from [center-width/2, center+width/2] to the nearest integer in [0,255]. For the MR image intensity values below (center-width/2), they are mapped to 0. Similarly, the image intensity values greater than (center+width/2) are mapped to 255. Apparently, these two parameters can greatly influence the appearance of the image to be displayed. In other words, the brightness and the contrast of an image is determined by these two parameters. Inadequate adjustment of these parameters can lead to degradation of image quality, and in severe cases to loss of valuable diagnostic information of the images.
Most previous methods for the adjustment of display window parameters are either very restricted to certain types of MR images or perform very differently from the human adjustment. R. E. Wendt III, "Automatic adjustment of contrast and brightness of magnetic resonance images", Journal of Digital Imaging, Vol. 7m No. 2, pp 95-97, 1994. Wendt III has proposed a method which first determines the type of an MR image by reading the image header information, and then computes the display parameters depending on the type of the image. Unfortunately, different rules must be set for different types and orientations of MR images in this method. This makes the algorithm impractical, since new rules need to be added in the program to reflect any new changes in the MR image acquisition process, such as, for example, the use of new coils or new pulse sequences. Ohhashi et al. has developed a neural network based method for the automatic adjustment of the display window. A. Ohhashi, S. Yamada, K Haruki, H. Hatano, Y Fujii, K Yamaguchi and H Ogata, "Automatic adjustment of display window for MR images using a neural network", Proceeding of SPIE, Vol. 1444, Omage Capture, Formatting and Display, pp. 63-74, 1991. This method is still a pure histogram based method, and as such, there is a potential problem of very different adjustments for images with very different spatial distributions but very similar histograms. In addition, this method only uses a single neural network for approximating the human adjustment, which is too complicated for a wide range of MR images to be sufficiently approximated with good generalization power by a single neural network.
Recently, the inventors proposed a comprehensive hierarchical neural networks (HNN) based algorithm for automatic and robust adjustment of the display window, which is the subject of U.S. patent application Ser. No. 08/885,080 entitled "Robust and Automatic Adjustment of Display Window Width and Center for MR Images" filed on Jun. 30, 1997, now U.S. Pat. No. 5,995,644, the entire disclosure of which is incorporated herein by reference. This algorithm is based on the principle of learning from examples, (i.e. a large set of MR images associated with the window width/center values adjusted by human experts). This HNN based algorithm uses both wavelet histogram features and spatial statistical information of MR images for feature generation, which overcomes the problem of using pure histogram information only. A hierarchical neural network was developed to decompose the very complicated function approximation problem into several simple subproblems. The hierarchical neural Rtworks are comprised of a modified competitive layer neural network for clustering any input image into a certain number of clusters, and the Radial Basis Function (RBF) and the Bi-modal Linear Estimation (BLE) networks for each class to provide good estimation results. Finally, a data fusion step is used to intelligently combine the multiple estimates from the RBF and BLE networks to provide accurate and robust estimation results.
All the above methods lack the capabilities of adapting the window width/center adjustment to different personal preferences or different viewing conditions as described above. The automatic display window width and center adjustment by using all the previous methods can only be optimized for a particular personal taste and for a particular viewing condition. The demand for an adaptive and automatic windowing system is very common since the automatic windowing system needs to be used by many persons with different personal preferences and in different viewing conditions.